The present invention relates to a microwave filter including at least two resonators, in which at least two degenerated wave types capable of resonance are provided in one of the resonators.
Such microwave filters are disclosed, for example, in IEEE MTT Volume 20, No. 15, April 1972, pages 258-265; IEEE MTT-32, No 11, November 1984, pages 1439-1454, or also in U.S. Pat. Nos. 3,697,898; 4,513,264 or 4,792,771. Such filters are characterized in that, several wave types are used simultaneously. In the above mentioned cases, the degenerated wave types are coupled in each resonator with one another, and the coupling is performed in correspondence with the main signal path (main coupling path). In other words, the wave types located in each resonator form electrically adjacent resonance circuits. In the case of the filter structure disclosed in the IEEE MTT, Volume 25, No. 12, December 1977, pages 1021-1026, the filter structure deals with a canonic form in which a main coupling between the wave types of a short-circuiting resonator is performed, while the coupling of the wave types inside the other resonator serves for overcoupling [transverse coupling] per each integral number of the resonance circuits.